Next-generation optoelectronic devices will require new materials and systems with characteristics not found in present-day materials. Traditional devices exploit metals, oxides, and semiconductors, each with their own functionality. For example, metals are typically formed to form contacts for application of a gate voltage or to supply current to a device. When high frequency electromagnetic radiation is involved, the functionality of the metal becomes more complex and obey plasmonics principles.
The field of plasmonics, which deals with the coupled oscillations of an electric charge and electromagnetic radiation, has found diverse applications in light localization, subwavelength focusing, etc., and has resulted in a wide range of devices.
Plasmonics has shown great potential for next generation devices which are based on strong electromagnetic field confinement. However, ohmic losses which originate in the cooling of excited (or so-called “hot”) carriers, have kept many devices from being developed and have led to a search for alternative materials.
Development of new ultrafast devices would be possible if carriers could be collected prior to cooling (or thermalization). Due to short diffusion lengths and scattering times of the carriers, devices are necessarily small (10 s-100 s of nm), and are thus excellent candidates for future miniaturized electronics.
Surface plasmon interactions can typically be divided into two cases: (a) localized surface plasmons and (b) propagating surface plasmon polaritons. For either case, an incident electromagnetic wave couples to the free charges in a metal and creates a coupled oscillation at the metal-dielectric interface. These oscillations are typically confined to a small volume and result in high field intensities. In addition, larger metallic particles (˜100 nm) may cause enhanced scattering of the incident light, while smaller particles (˜10 s nm) cause enhanced absorption within the metal particles. The ability of metallic nanostructures to effectively confine and scatter light has led to many applications of plasmonics to photodetectors.
Manipulation of light-matter interactions enabled functionally that surpasses the limitations of traditional materials for applications such as optical cloaking, water splitting for hydrogen production, and optical energy conversion, has been of great interest in the past decade. In particular, controlling transmission and reflection from material interfaces can improve optical coatings for filters and the absorption efficiency of photodetectors and solar cells. This desire for optical control has led to the development of metamaterials, which exhibit optical properties that are not found in nature. Metamaterials that enhance localized electric fields through exciting plasmonic resonances in metallic nanostructures have been developed to increase absorption and extend the bandwidth of semiconductors. However, these metamaterials require complicated and costly nanofabrication techniques making them difficult to commercialize.
Recently, high absorption in ultra-thin films has been theorized and experimentally demonstrated by exciting zeroth order Fabry-Perot (FP) resonances. These resonant cavities combine interference effects and phase delay to obtain high absorption and are a cost effective alternative to plasmonic metamaterials because of their ease of fabrication. While zeroth order FP cavities have been used to increase absorption in ultra-thin semiconductors above their bandgap, and their generated photocurrent has been harnessed for water splitting, they have not been utilized for hot carrier generation in metals.
Hot carrier devices such as plasmonic metamaterial absorbers and nano-antenna arrays have successfully generated a photoresponse from sub-bandgap photons in silicon. However, they require complicated and costly nanofabrication techniques that limit their advantage over low bandgap semiconductors.
Silicon-based CMOS image sensors are pervasive and found in many consumer electronics (e.g., cell phones, cameras, etc.). While these sensors allow for imaging of visible light, they are unable to capture the infrared (IR) light due to the mismatch with the bandgap of silicon (Si). In order to image radiation in the infrared spectrum, a separate image sensor is needed, typically based on such semiconductors as Ge, InGaAs, etc. The necessity of a separate IR image sensor interferes with miniaturization of the image sensor, and complicates the fabrication process.
It is economically advantageous to explore methods of photodetection using silicon in the IR/telecommunications regime, below its energy bandgap, because of the abundance and relatively low cost of silicon compared to lower bandgap semiconductors.
It would be highly advantageous to utilize principles of plasmonics and, particularly, hot carriers generation and injection from ultrathin metal films formed in contact with silicon to obtain a photoresponse to IR radiation.
It also would be highly desirable to provide a hot carriers based photodetection system capable of expanding the bandwidth of silicon detectors into the Infrared Spectrum by generating photocurrent from photons with energy below the silicon bandgap which would be advantageous for many applications requiring optical energy conversion due to the fact that silicon is a well-understood, naturally abundant and inexpensive material for electronic systems.